The goal of this project is to understand the roles of local feedback systems to the cochlear nuclei in shaping the neural representation of sound for processing by higher brain centers. Three local feedback systems which reside in the brainstem have been identified. These are: (1) the system controlling the middle ear muscles, including pathways subserving the middle ear muscle reflex; (2) the system which projects from the superior olive to both the cochlea and cochlear nuclei, the so-called olivo-cochlear system; and (3) the system which projects from the superior olive exclusively to the cochlear nuclei. The cells which give rise to the latter two systems are scattered in groups called peri-olivary nuclei (PON), which surround the principal cell groups of the superior olive. Cells of the PON receive direct projections from the cochlear nuclei, and project to diverse auditory brainstem nuclei. One cell group, the lateral nucleus of the trapezoid body (LNTB), consists of cells which project heavily to the ipsilateral cochlear nucleus. Therefore, the LNTB represents a well segregated part of the local feedback to the cochlear nuclei and has a relatively simple organization. This project is a focused study of the LNTB and may reveal principles which can aid in understanding the roles of other descending projections to the cochlear nuclei. A description of the extent to which the LNTB is influenced by input from the cochlear nuclei, as opposed to inputs from higher brain centers, will be the aim of many experiments in this proposal. The experiments are guided by the hypothesis that feedback projections play a role in preserving the representations of sound in the presence of noisy backgrounds, and can work by the mechanism of affecting lateral inhibition of cells of the cochlear nuclei. The experiments are divided into two groups. The first group of experiments is designed to determine the anatomical organization and physiological properties of LNTB neurons. The second group of experiments test the hypothesis of LNTB function. The proposed experiments use a combination of in vitro (brain slice) and in vivo electrophysiological and anatomical techniques that are already in use in the laboratory. The combination of preparations will bring greater experimental control and flexibility to bear on investigation of LNTB functions and mechanisms. These experiments may reveal neural mechanisms of signal extraction in a noisy background, which may also provide information important for improving designs of prosthetic devices that replace nonfunctioning parts of the auditory nervous system.